The invention discloses an online monitoring system for ultrahigh-frequency partial discharge of high-voltage parallel reactors of a substation. The two ends of a metal shell of the online monitoring system are respectively connected with a cover plate and an epoxy resin plate to form a closed chamber. A shielding plate divides the chamber into two closed spaces. A fixed bracket is arranged on the inner wall of the metal shell in the lower space. An amplifier plate is installed on the fixed bracket. An antenna plate is installed on the inner surface of the epoxy resin plate. An aviation plug and an N-type connector are arranged on the wall of the metal shell in the upper space. The antenna plate, the amplifier plate, the aviation plug and the N-type connector are sequentially and serially connected through conducting wires. The lower end of the metal shell is fixedly connected with a circular ring plate. The online monitoring system for ultrahigh-frequency partial discharge of the high-voltage parallel reactors of the substation has the advantages that the external electromagnetic signal shielding effect is very good, the amplifier plate and the antenna plate in the shell can be enabled to more accurately receive electromagnetic waves produced by partial discharge in the high-voltage parallel reactors and the installation is facilitated; the anti-interference capacity is strong, the signal bandwidth is wide, the dynamic range is wide and the sensitivity is high.